Rotary helically splined actuators have been employed in the past to achieve the advantages of high-torque output from a simple linear piston and-cylinder drive arrangement. The actuators typically employed a cylindrical body with an elongated rotary output shaft extending from end to end coaxially within the body, with an end portion of the shaft providing the drive output. Disposed between the body and the shaft is a piston sleeve, having a sleeve splined to cooperate with corresponding splines on the body interior and the output shaft exterior. The piston sleeve is reciprocally mounted within the body and has a piston head for the application of fluid pressure to one or the other opposing sides thereof to produce axial movement of the piston sleeve. The sleeve is elongated and coaxially receives the shaft therein.
As the piston sleeve linearly reciprocates in an axial direction within the body, the outer splines of the sleeve engage the splines of the body to cause rotation of the sleeve. The resulting linear and rotational movement of the sleeve is transmitted through the inner splines of the sleeve to the splines of the shaft to cause the shaft to rotate. Bearings are typically supplied to rotatably support one or both ends of the shaft relative to the body.
With such an arrangement, as the piston sleeve reciprocally moves from one axial direction to the other to produce relative rotational movement between the body and the shaft in response to the application of fluid pressure to the piston head, backlash results from the slack existing between the intermeshing splines of the sleeve and the body and the intermeshing splines of the sleeve and the shaft. While accurate machining of the splines will reduce the backlash problem, this procedure substantially increases the manufacturing cost. Even with accurate machining, conventional machining techniques are virtually incapable of totally eliminating the slack which produces the backlash problem. Furthermore, to the extent more accurate tolerances produce actuator parts which fit tightly together and reduce slack, assembly of the actuator becomes difficult. While accurate machining reduces slack initially, should the spline parts wear during usage or otherwise lose their original tolerances, no means exist for elimination of the slack that develops without remachining or replacement of the splined parts.
It will therefore be appreciated that there has been a significant need for a fluid-powered rotary actuator which does not require exceptionally accurate machining of the torque-transmitting parts to eliminate slack that produces backlash. The actuator should be easy to assemble and provide means for substantially complete elimination of the slack causing the backlash problem after the actuator components with the slack are assembled. Elimination of the slack should be accomplished in a simple manner without requiring disassembly of the torque-transmitting parts. Furthermore, the means for elimination of the slack should, with one adjustment, simultaneously equally remove the slack existing between all of the torque-transmitting parts between the piston sleeve and the shaft which translate linear movement of the piston sleeve into rotational movement of the shaft. It is desirable that the actuator have a very low weight and large torque-to-weight ratio while providing rotation of up to 180 degrees or more. The present invention fulfills these needs and further provides other related advantages.